A mask thickness monitoring device which measures the thickness of an etching mask (object of measurement) using interference light is known in the prior art (for example, see Patent Literature 1).
The aforementioned Patent Literature 1 discloses a mask thickness monitoring device comprising a tungsten lamp which irradiates a specimen, wherein an etching mask has been formed on a silicon wafer, with measurement light; a spectrometer which detects interference light due to light reflected from the etching mask and outputs a spectrometric spectrum; and a mask thickness computation unit which computes the mask thickness based on curve fitting of a theoretical waveform and a spectrum waveform of the interference light intensity-wavelength obtained from the spectrometer. It will be noted that the tungsten lamp is a wide band light source which produces white light.